1. Field of the Invention
The present invention relates to a method of making a parallax barrier. The present invention also relates to a parallax barrier made by such a method and an autostereoscopic three dimensional (3D) display including such a barrier. Uses of such barriers include consumer and professional photography and uses of such displays include 3D television, police identification, medical imaging, scientific visualisation, point of sale counters and 3D design.
2. Description of the Related Art
FIG. 1 of the accompanying drawings illustrates a known type of autostereoscopic 3D display of the front parallax barrier type. The display comprises a liquid crystal spatial light modulator 1 arranged to provide a plurality of picture elements (pixels) such as 2. The pixels are arranged as vertical columns and display two images as interlaced vertical strips. A parallax barrier 3 is provided on a front surface of the device 1 and is spaced from the plane containing the pixels 2. A light source (not shown) is provided behind the rear surface of the device 1 for illuminating the pixels 2.
The parallax barrier 3 restricts the visibility of the columns of pixels at a designed viewing distance in front of the display so as to form viewing windows such as 4. The horizontal pitch of the vertical slits such as 5 of the parallax barrier 3 is slightly less than twice the horizontal pitch of the pixel columns so as to provide view point correction as illustrated in FIG. 2. Within a left eye viewing “diamond” 6, the columns of pixels displaying an image for viewing by the left eye of an observer are visible whereas the other columns are not visible. Conversely, within a right eye viewing “diamond” 7, the columns of pixels displaying an image intended for viewing by a right eye of the observer are visible whereas the other columns of pixels are not visible. Thus, provided an observer observes the display with the left and right eyes located within the left and right eye viewing diamonds 6 and 7, respectively, the observer can perceive a 3D image. The viewing windows are formed at the laterally widest regions of the viewing diamonds 6 and 7 such that the observer has the greatest degree of lateral freedom of movement while seeing the 3D image when the eyes are at the viewing windows. The width of each viewing window is designed to be substantially equal to the average interocular separation so as to provide the maximum amount of lateral viewing freedom.
In a ideal display, the intensity distribution of light across each viewing window would be a “top hat function” such that, when an eye is in the window, it sees maximum light intensity which is constant across the viewing window whereas zero intensity would be seen by an eye outside the viewing window. This is illustrated in FIG. 3 of the accompanying drawings, which illustrates left and right eye viewing windows at 10 and 11, respectively, the ideal left eye intensity function with respect to lateral position at 12, and the actual “non-ideal” window function which is generally achieved at 13. As shown by the actual function 13, light intended for the left eye viewing window 10 has an intensity which varies somewhat with lateral position within the window and which does not fall abruptly to zero at the edge of the window but instead slopes down to a non-zero intensity in the region of the right eye viewing window. The non-ideal function results, for example, from diffraction in the slits of the parallax barrier. Thus, lateral viewing freedom is reduced and a small amount of light from the left eye image is visible to the right eye and vice versa resulting in crosstalk. The design of the parallax barrier slit width in such displays is a compromise between wide slits, which allow a high light throughput but give high crosstalk, and narrow slits, which give reduced crosstalk but suffer from low brightness.
EP 0 822 441 discloses a technique for reducing diffraction effects from pixel apertures in rear-illuminated autostereoscopic displays. This technique involves varying the pixel aperture function and providing grey scale modification of the edges of the parallax barrier slits.
EP 1 072 924 discloses a technique for reducing diffraction from parallax barriers in both front and rear parallax barrier displays. This technique involves forming the slits as multiple sub-apertures of varying intensity.
Montgomery et al “Analysis of the performance of a flat panel display system convertible between 2D and autostereoscopic 3D modes”, Proc SPIE, vol 4297, January 2001, ISSN 0277-786X describes a theoretical model based on Fresnel diffraction theory with predictions of window shapes and crosstalks which match experimental evidence to a high degree of accuracy.
These documents describe the difference between a “hard edge” parallax barrier and a “soft edge” parallax barrier. FIG. 4 illustrates the function of transmissivity against position across a slit of hard edge and soft edge barriers. In the hard edge barrier, there is a sharp transition between the opaque area and the transparent area of the barrier whereas, in the soft edge barrier, there is a more gradual change in transmissivity between the opaque and transparent areas at the edges of the barrier slits.
EP 1 072 924 also discloses a technique for manufacturing a soft edge parallax barrier having a plurality of sub-apertures. In this technique, a hard edge mask having a plurality of slits for forming each parallax barrier slit is spaced from a photographic material. The material is then exposed to light passing through the mask from a light source, after which the material is developed to form the parallax barrier. In order for this technique to work, knowledge of the diffraction profile and control of the light source are required. Also, relatively precise knowledge of the photographic material, such as an emulsion on a substrate, and its grey scale response is required.
Other known techniques for producing hard edge parallax barriers are described in GB 1 057 105, DE 2 501 195, JP 6 301 5249, and RA Lawes “Future developments for optical mask technology”, Microelectronic Engineering 23 (1-4) 1994, pp 23-9, ISSN 0167-9317.